Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; a protective layer or slip film; and a cover sheet.
The backing layer lends support to the plate, and is typically a plastic film or sheet, which may be transparent or opaque.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a solid composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. If a second photocurable layer is used, i.e., an overcoat layer, it typically is disposed upon the first layer and is similar in composition.
The photocurable materials generally cross-link (cure) and harden in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and infrared wavelength regions. Preferred actinic wavelength regions are from about 250 nm to about 450 nm, more preferably from about 300 nm to about 400 nm, even more preferably from about 320 nm to about 380 nm. One suitable source of actinic radiation is a UV lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin sheet, which protects the photopolymer from dust and increases its ease of handling. In a conventional plate making process, the slip film is transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
In “digital” plate making processes, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative on a digital (i.e., laser ablatable) masking layer, which is generally a modified slip film (i.e., a slip film layer which has been doped with a UV-absorbing material, such as carbon black). Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser.
The laser ablatable layer can be any photoablative masking layer known in the art. Examples of such laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. The laser ablatable layer generally comprises a radiation absorbing compound and a polymeric binder. The radiation absorbing compound is chosen to be sensitive to the wavelength of the laser and is generally selected from dark inorganic pigments, carbon black, and graphite.
The polymeric binder is generally selected from polyacetals, polyacrylics, polyamides, polyimides, polybutylenes, polycarbonates, polyesters, polyethylenes, cellulosic polymers, polyphenylene ethers, polyethylene oxides, and combinations of the foregoing, although other suitable binders would also be known to those skilled in the art. The binder is selected to be compatible with the underlying photopolymer and easily removed during the development (wash) step. Preferred binders include polyamides, and cellulosic binders, such as hydroxypropyl cellulose.
The benefit of using a laser to create the image is that the printer need not rely on the use of negatives and all their supporting equipment, and can rely instead on a scanned and stored image, which can be readily altered for different purposes, thus adding to the printer's convenience and flexibility. When a negative is used, the slip film has to be transparent to the light used for curing. Since UV flood lamps normally provide the light for curing, the normal slip film is transparent in the range of 300–400 nm. Such films are well known in the photoprocessing field, and in principle, any such film may be modified by adding a suitable radiation-absorbing compound.
After imaging, the photosensitive printing element is developed to remove the masking layer and the unpolymerized portions of the layer of photocurable material to create a relief image on the surface of the photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter.
Printing plates with laser ablatable masks can be used to form semi-continuous imaging elements. The flat sheet elements are cut to size and wrapped around a cylindrical form, usually a printing sleeve or the printing cylinder prior to imaging of the laser ablatable (digital) mask by a laser. A longitudinal seam is created by this process, which is often referred to as “plate-on-sleeve.” This seam can be straight across the surface of the sleeve or cylinder, or can be made in an infinite variety of shapes through the use of manual or automated cutting methods. A common practice is to “stagger” the seam through a “stair step” pattern. “Staggered” seams are used to minimize rotational balance problems that can be caused by straight seams. Another practice that can be used to create circumferential seams is the use of separate plates arrayed in “lanes” over the cylinder. In this case, there are seams not only at the ends of each plate wrapped around the cylinder, but also between each lane. If care is not taken to cover the photocurable surfaces exposed by the cutting process with a material that is opaque to the UV radiation used to expose the plate, a phenomenon called “edge cure” can result.
Edge cure is caused by UV light contacting the cut edges and corners of the plate, which polymerizes the photopolymer and creates an undesirable raised border around the edges of the plate. This border must then be manually cut from the plate, which requires time and can result in damage to the plate, especially if portions of the images are near the plate edge. In addition, removal of the raised border may leave an undesirable residue on the plate, which must also be removed.
One current process used to prevent edge curing uses a felt tip pen that contains a UV-opaque ink as a means of sealing the edges of such plates. However, this is a slow, tedious process that is only about 90 percent effective at preventing edge curing of the plate.
Another process is described in U.S. Pat. No. 6,326,124 to Alince et al., the subject matter of which is herein incorporated by reference in its entirety. Alince et al. discloses an edge-covering material consisting essentially of at least one soluble, film-forming polymer, at least one UV absorber, and a solvent or solvent mixture that is applied on the edges of a photocurable printing plate before imagewise exposure of the printing plate to prevent unwanted ridges that result from exposure of printing plate edges. The edge-covering material is applied by brushing or spraying, preferably with a fine nozzle. Typical solvents are volatile solvents having a high solvent capability for the film-forming polymer, including toluene, xylene, methyl ethyl ketone, and ethyl acetate. However, with a solvent-borne formulation it is extremely difficult to remove the edge-covering material without damaging the imaging mask if it is accidentally applied to portions of the printing plate intended for imaging.
Thus, there remains a need in the art for improved methods of treating cut surfaces (i.e., edges and corners) of printing plates to prevent the formation of unwanted ridges on the edges of the plate and for a method that can be performed more easily and is more fault-tolerant than processes described in the prior art.
The inventors have surprisingly discovered that conventional, commercially available sunscreen formulations (or compositions containing similar ingredients) are highly effective edge-cure prevention agents. Furthermore, these formulations are easy to apply, non-toxic, inexpensive, and are largely compatible with the solvents used to wash uncured photopolymer from the printing plates during processing. In addition, such formulations can be easily wiped off the plate surface if necessary, with no damage to the digital mask. The lighter color of the sunscreen formulation also makes it easy to visually discern which areas of the edge are completely covered by the formulation.
The formulations of the invention are coated onto the edges of a printing plate after the printing plate has been cut to the desired size and shape. The improved process of the invention is more fault-tolerant as compared with comparable processes of the prior art. In addition, the formulations of the invention are non-toxic, and require no special precautions for handling or avoiding skin contact.